Table 2.
The oxidative potential of TiO2
| Paper | Particle | Model | Endpoints Assessed | Observation | Conclusion |
|---|---|---|---|---|---|
| Afaq et al., [46] | TiO2 (<30 nm) | Response of primary alveolar macrophages (following intratracheal exposure of rats) | Glutathione peroxidase, glutathione reductase, glutathione-s-transferase activity Intracellular GSH Lipid peroxidation (thiobarbituric acid reactive substances measured) H2O2 production Cytotoxicity (LDH assay) |
Decreased GSH Increased lipid peroxidation Increased H2O2 (indicative of respiratory burst) Increased glutathione peroxidise & glutathione reductase Decreased cell viability |
An oxidant driven inflammatory, and cytotoxic response was observed within macrophages on exposure to TiO2 |
| Dunford et al., [58] | TiO2 (extracted from commercially available sunscreens) | DNA oxidative damage (plasmid DNA & within MRC-5 fibroblasts) | Oxidation of organic material (phenol) Plasmid DNA (in vitro) Comet assay (MRC-5 cells) (all experiments conducted in sunlight illuminated conditions) |
TiO2 stimulates oxidation of organic materials (due to production of hydroxyl radicals) & strand breaks in plasmid DNA. DNA damage decreased with free radical quenchers (mannitol & DMSO) - illustrates that it is oxidant driven DNA damage observed in comet assay & is oxidant driven |
Oxidative damage to DNA by TiO2 |
| Gurr et al., [28] | TiO2 (10, 20 or >200 nm) | BEAS-2B epithelial cells | Oxidative DNA damage (Comet assay) Lipid peroxidation (MDA) NO and H2O2 production Cell viability (MTT assay) |
Increased DNA damage Increased lipid peroxidation Increased NO & H2O2 Decreased cell viability Responses only for 10 & 20 nm NPs |
Oxidative stress induced appears to be size dependent, and has genotoxic and cytotoxic consequences |
| Jin et al., [35] | TiO2 (20-100 nm) | L929 fibroblasts | Cell viability (MTT DH assays) ROS production (dichlorofluorescein (DCFH) assay) GSH & SOD cell levels |
Decreased cell viability Increased ROS production Decreased GSH and SOD |
TiO2 mediated oxidative stress is related to a loss of cell viability |
| Kang et al., [49] | TiO2 (21 nm & 1 μm) | RAW 264.7 macrophages | Intracellular ROS generation (DCFH assay & dihydroethidium staining) Cell viability (LDH) Cytokine production MAPK signalling pathway activation |
No loss in cell viability Increased ROS production (greater for NPs) Increased MIP-2 and TNFα ERK1/2 phosphorylation (part of MAPK pathway) |
NPs stimulate the production of ROS that, in turn activate a signalling cascade (involving ERK1/2) to promote the development of an inflammatory response |
| Karlsson et al., [57] | CuO (42 nm), ZnO (71 nm), TiO2 (63 nm), Fe3O4 (20-30 nm) | A549 lung epithelial cells | Cell viability (trypan blue) ROS production (DCFH assay) Comet assay |
Cytotoxicity greatest for CuO CuO increased ROS and elicit DNA (oxidative mediated) damage -Fe3O4 did not elicit toxicity |
CuO most toxic NP, via an oxidative mechanism, but the release of ions may be responsible for the observed toxicity Metal oxide NPs vary in their ability to elicit oxidant mediated damage |
| Long et al., [43] | TiO2 | BV2 microglia, N27 neurones | ROS production (DCFH) H2O2 production (Image-IT LIVE fluorescent probe) Superoxide production (MitoSOX fluorescent probe) Apoptosis (capase 3/7 activity & nuclear staining) |
Increased ROS production Increased H2O2 (rapid response, 1-5 mins) Increased superoxide (later response, 30 mins onwards) Increased Apoptosis -Toxicity only evident in BV2 cells |
Neurotoxicity mediated by TiO2 is oxidant mediated Cell dependent sensitivity to toxicity observed. |
| Lu et al., [51] | TiO2 | BSA | Protein nitration (detected spectrophotomically & western blotting) (experiments conducted with UV irradiation) |
Protein nitration is crystal form dependent Antioxidants prevent against protein nitration |
Protein nitration is crystal form and light dependent |
| Park et al., [26] | TiO2 (21 nm) | BEAS-2B lung epithelial cells | Cell viability (MTT assay) ROS production (DCFH assay) GSH depletion Apoptosis (caspase-3 assay & chromosome condensation) Gene expression (RT-PCR) |
Increased cytotoxicity Increased ROS production Decreased GSH Increased apoptosis Increased expression of oxidative stress (e.g. catalase, HO-1, glutathione-S- transferase) & inflammatory (IL-1, IL-6, IL-8, TNFα) genes |
TiO2 NPs induce oxidative stress in cells, which is responsible for the observed inflammatory & cytotoxic (via apoptosis) responses |
| Sayes et al., [71] | TiO2 (in various crystal forms) | HDF (dermal fibroblasts) & AA549 (lung epithelial) cells | Cytotoxicity (LDH, MTT & live/dead assays) Inflammation (IL-8 production) Particle suspension ROS ex vivo production |
Increased cytotoxicity Increased ROS (ex vivo) production Increased IL-8 production -Response dependent on crystal form |
Toxicity exhibited by TiO2 is phase dependent, and involves, oxidative, inflammatory and cytotoxic components |
| Wang et al., [17] | TiO2 (in rutile (80 nm) & anatase (155 nm) forms) | Nasal Instillation (mice) |
Enzyme activity (gluthathione peroxidise, catalase, SOD, glutathione-S-transferase) GSH levels Lipid peroxidation (MDA) Protein oxidation (protein carbonyl formation) (All responses evaluated in the brain) |
Increased MDA Increased catalase Decreased SOD Increased protein oxidation -No changes in other markers |
TiO2 distributes within the brain and elicits oxidative damage, which is dependent on the crystal phase of the particles |
| Xia et al., [50] | TiO2 (11 nm) (also ZnO (13 nm) & CeO2 (8 nm)) | RAW 264.7 macrophages & BEAS-2B lung epithelial cells | Cytotoxicity (Propidium iodide & MTS assays) Intracellular ROS production (DCFH assay), and HO-1 antioxidant expression. Pro-inflammatory signalling cascade activation (nfKB) and intracellular calcium concentration. Cytokine production (TNFα & IL-8) |
No increase in cytotoxicity, ROS generation or inflammation was observed | The most toxic particle in the panel was ZnO. Toxicity was absent for TiO2. |